Measuring Horizontal Shear and Turbulence in Mountain Valleys using UAS and Lidar

Turbulent mixing in complex terrain introduces significant uncertainty in weather and climate models, as critical processes within the mountain boundary layer (MoBL) are not represented. While common mesoscale models neglect the horizontal shear production of turbulent kinetic energy (TKE), this assumption is invalid in mountainous regions, necessitating 3D boundary layer parameterizations including these contributions. However, observational data quantifying horizontal versus vertical shear production remains scarce. To address this gap, we deployed a measurement strategy combining small uncrewed aircraft systems (UAS) and remote sensing systems, notably Doppler Wind Lidar (DWL), during the TEAMx (Multi-scale transport and exchange processes in the atmosphere over mountains - programme and experiment) 2025 campaign the Inn Valley in Austria. Commercially available and automatically operating multi-rotor UAS equipped with fast-response sensors measured temperature, humidity and aerosols, as well as the 3D wind vector up to 4 Hz, resolving eddies of 2.5 m at 10 ms−1 mean wind speed. Four UAS simultaneously recorded vertical profiles up to 2 km above mean sea level and horizontal cross-sections across key valley locations (valley floor, sidewall foot, slope, and crest), with the spacing representative of targeted operational weather forecast grids. This was coordinated with three DWL systems providing continuous wind profiles at the same locations. The combined measurements enabled an analysis of spatially and temporally resolved MoBL dynamics. For thermally driven up-valley flows, TKE increases horizontally from the valley center toward the mountain, peaking vertically near the mountain ridge. This observed rise in TKE coincides with strong horizontal wind shear up to 10 ms−1 wind speed difference per km. By integrating UAS and remote sensing observations with model development in the TEAMx framework, we aim to advance physically consistent turbulence parameterizations for high-resolution numerical weather simulations, enhancing forecast reliability in complex terrain.